changed initStatic?Dict() return type to const ZSTD_?Dict*

ZSTD_create?Dict() is required to produce a ?Dict* return type
because `free()` does not accept a `const type*` argument.
If it wasn't for this restriction, I would have preferred to create a `const ?Dict*` object
to emphasize the fact that, once created, a dictionary never changes
(hence can be shared concurrently until the end of its lifetime).

There is no such limitation with initStatic?Dict() :
as stated in the doc, there is no corresponding free() function,
since `workspace` is provided, hence allocated, externally,
it can only be free() externally.

Which means, ZSTD_initStatic?Dict() can return a `const ZSTD_?Dict*` pointer.

Tested with `make all`, to catch initStatic's users,
which, incidentally, also updated zstd.h documentation.

lib/decompress/zstd_decompress.c

@@ -2139,9 +2139,10 @@ ZSTD_DDict* ZSTD_createDDict_byReference(const void* dictBuffer, size_t dictSize
 }
 
 
-ZSTD_DDict* ZSTD_initStaticDDict(void* workspace, size_t workspaceSize,
-                                 const void* dict, size_t dictSize,
-                                 ZSTD_dictLoadMethod_e dictLoadMethod)
+const ZSTD_DDict* ZSTD_initStaticDDict(
+                                void* workspace, size_t workspaceSize,
+                                const void* dict, size_t dictSize,
+                                ZSTD_dictLoadMethod_e dictLoadMethod)
 {
     size_t const neededSpace =
             sizeof(ZSTD_DDict) + (dictLoadMethod == ZSTD_dlm_byRef ? 0 : dictSize);